A process for upgrading plastic material containing inorganic filler or reinforcement material for use as feedstock in partial oxidation gas generator for the production of raw synthesis gas, fuel gas, or reducing gas. The plastic material is granulated and mixed with water to produce the plastic sludge. The plastic sludge is preheated at a temperature in the range of about 350.degree. F. to 475.degree. F. in the absence of air in a closed system. The preheated plastic sludge is then hydrothermally treated at a temperature in the range of about 450.degree. F. to 650.degree. F. at a pressure in the range of about 100 to 1200 psig and above the vapor pressure of water at that temperature. The hydrothermally treated plastic sludge is cooled, degassed, and mixed with ground solid carbonaceous fuel, e.g., coal and water, to produce a pumpable aqueous slurry having a solids content in the range of about 40 to 60 wt. %. The pumpable aqueous slurry is then reacted by partial oxidation to produce said synthesis gas, fuel gas, or reducing gas. In one embodiment, a portion of coal is mixed with the plastic sludge prior to the aforesaid preheating and hydrothermal steps.